Bradykinin (BK), a nonapeptide, originally discovered in blood, is also present in neurons and fibers of the hypothalamus. We tested the putative releasing factor properties of BK on prolactin (PRL) release from anterior pituitary cells in vitro. BK stimulated the release of PRL in a dose-dependent manner, the threshold concentration being in the range. 0.1-1.0 nM. The release of PRL induced by BK at 1 nM concentration was about 2-fold, delayed and sustained over many minutes. Higher concentrations of BK stimulated PRL release in two phases. The shape of the BK-induced PRL release was superficially similar to that induced by thyrotropin-releasing hormone (TRH). 10 nM BK and 10 nM TRH induced about a 4-fold increase in PRL release within 5 min, followed by a gradual recovery to basal secretion. These results indicate that this peptide can act directly at the anterior pituitary gland to release PRL. Phorbol ester also promoted PRL release over the range of 1-10 nM, but the time course of the release was somewhat different.